Current transformers especially zero-type transformers such as large ground fault interrupters (GFI), utilize cores exhibiting a rounded (sheared over) B-H loop along with a high saturation induction. Distribution transformers require cores having a square B-H loop and low core loss and exciting power. Cores for both types of transformers are typically large. Distribution and zero-type current transformers range from 1 to 100 kg, and have an OD of up to about 600 mm.
It is necessary to field anneal the cores to achieve the B-H loop shapes which are required for these applications. For small cores (less than about 100 g) permanent magnets may be used. Presently, only magnets which are made from AlNiCo or Sm/Co can withstand the furnace temperatures necessary for annealing. Such magnets are extremely difficult to make and magnets with flat faces which are larger than about 4 inches (10 cm) are not known to be available. Accordingly, for cores with inner and outer diameters (ID and OD respectively) which are larger than about 4 inches (10 cm), the use of permanent magnets is not practical. Moreover, large cores are conventionally annealed in large furnaces under protective atmospheres. Annealing under such conditions is very costly because large cores can take as long as eight hours or more to anneal.
Resistance annealing an amorphous alloy wound core by flowing a current through a coil disposed about a leg of the core (thereby generating heat due to energy loss in the core) is disclosed in Japanese application Nos. 85/161,497 and 85/135,301, both filed on Jun. 21, 1985. Similarly, Japanese application No. 84/181,850 discloses the production of an iron core for transformers by making a core from a magnetic amorphous alloy plate, winding coils around the core and exciting the core to anneal.
Accordingly, there remains a continuing need for a practical method for field annealing large cores particularly current transformer cores, which does not require the use of a protective atmosphere, long annealing times, or the development of new magnets.